This invention relates to vehicle obstruction detection systems and particularly, but not exclusively, to such detection systems for use on vehicles in car parks and other areas of restricted space and random `obstructions`.
When reversing a vehicle into a confined space or up to a stationary object such as another vehicle, wall, lamp-post etc, it is frequently difficult, from the position of the driver's seat, to estimate the clearance distance between the rear of the vehicle and adjacent objects. Such objects may be damaged by, or be a hazard to, the vehicle.
Radar systems for detecting the presence and range of objects are of course well known. Similarly, light based systems using lasers for the same purpose--"lidar"--are known. In such systems the range is commonly determined by means of a series of "range gates" which open for a predetermined short time commencing at some predetermined time after the transmission of the radar pulse. The delay between transmit pulse and gate opening is staggered so that the range gates cover successive periods in the range of interest, and thus cover the actual range of interest. A complete picture in the direction of transmission can thus be obtained from the various returns of a single pulse from different range cells. Such a system may be satisfactory in most radar situations, eg. where the target is a tank, aircraft or other military entity at some distance, but presents difficulty when the circumstances are of a much more localised or domestic nature and particularly when the `target` range is only several meters at maximum and more commonly only tens of centimeters.
In such circumstances the pulse transit time is likely to be of the order of a nanosecond or less and each range cell perhaps 15 centimeters. Processing circuitry which could cope with successive signal samples at intervals of a nanosecond would at best be very expensive and at worst be impracticable. Anologue-digital converters in particular are generally not able to operate at this speed.
In another context it is known to use radar to determine the height of clouds, to assist in the landing and take-off of aircraft for example. A similar principle is involved in that radar pulses are transmitted vertically upward and range gates are timed to receive signals from successive range cells. In this case however, the maximum range could be in the region of 3000 meters with range cells of perhaps 20 meters. The assessment of a single range cell per pulse (or per multiplicity of pulses) has been proposed but there is clearly no pressure on the speed of the processing electronics in view of the great distances and substantial transit times involved.